Filter circuit and radio apparatus

ABSTRACT

A distributor  103  distributes an input signal into two lines of distributed signals with equal amplitudes and equal phases. Buffers  104, 105  suppress interference between distributed signals distributed into the two lines. A filter  106  performs a frequency selection which allows only a distributed signal in a predetermined band to pass. A differential amplifier  107  outputs a difference in amplitude components between a distributed signal frequency-selected by the filter  106  and the distributed signal which is not frequency-selected. This allows an attenuation characteristic or passage characteristic in a high-frequency band to be maintained.

TECHNICAL FIELD

The present invention relates to a filter circuit and radio apparatus,and more specifically, to a filter circuit and radio apparatus for ahigh-frequency band used for a digital radio communication system, forexample.

BACKGROUND ART

Some conventional filter circuits using an amplifier realize a filtercharacteristic using conductance of the amplifier and capacitance.According to this filter circuit, a positive-phase input terminal of afirst conductance amplifier, which constitutes an active filter, isconnected to a first input terminal via first resistor and connected toa ground point via second resistor. Furthermore, a negative-phase inputterminal of the first conductance amplifier is connected to a secondinput terminal via third resistor and connected to an external outputterminal via fourth resistor. Then, the resistance value ratio betweenthe first resistor and second resistor is equalized to the resistancevalue ratio between the third resistor and fourth resistor and onesignal including an in-phase signal component is added to the firstinput terminal and the other signal is added to the second inputterminal. In this way, the conventional filter circuit adopts amulti-stage structure with each stage consisting of a capacitance andconductance amplifier, and can thereby obtain a steep attenuationcharacteristic.

However, the conventional filter circuit and radio apparatus realizenothing more than a filter in a low pass type configuration, andrealizing a band pass type configuration in a high-frequency bandrequires a combination of a low pass type configuration and high passtype configuration, and furthermore, obtaining a steep attenuationcharacteristic requires a multi-stage configuration, and there areproblems that the number of capacitances and conductance amplifiersincrease and the size of the circuit increases, making integration ofthe circuit difficult. On the other hand, reducing the number of stagesof the filter circuit to reduce the amount of attenuation of a desiredfrequency component without increasing the size of the circuit resultsin a problem that it is difficult to realize a steep attenuationcharacteristic or passage characteristic in a high frequency band.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a filter circuit andradio apparatus capable of maintaining an attenuation characteristic orpassage characteristic in a high-frequency band using an in-phaseelimination effect of a differential amplifier.

This object can be attained by distributing an input signal into twolines of distributed signals, applying a frequency selection to one ofthe distributed signals and outputting a difference in amplitudecomponents between the one frequency-selected distributed signal and theother not frequency-selected distributed signal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration of a filter circuitaccording to Embodiment 1 of the present invention;

FIG. 2 illustrates a power-frequency relationship of a signal accordingto Embodiment 1 of the present invention;

FIG. 3 illustrates a power-frequency relationship of a signal accordingto Embodiment 1 of the present invention;

FIG. 4A illustrates a power-frequency relationship of a signal accordingto Embodiment 1 of the present invention;

FIG. 4B illustrates a power-frequency relationship of a signal accordingto Embodiment 1 of the present invention;

FIG. 4C illustrates a power-frequency relationship of a signal accordingto Embodiment 1 of the present invention;

FIG. 5A illustrates a power-frequency relationship of a signal accordingto Embodiment 1 of the present invention;

FIG. 5B illustrates a power-frequency relationship of a signal accordingto Embodiment 1 of the present invention;

FIG. 5C illustrates a power-frequency relationship of a signal accordingto Embodiment 1 of the present invention;

FIG. 6 illustrates a power-frequency relationship of a signal accordingto Embodiment 1 of the present invention;

FIG. 7 illustrates a power-frequency relationship of a signal accordingto Embodiment 1 of the present invention;

FIG. 8 is a block diagram showing a configuration of a filter circuitaccording to Embodiment 2 of the present invention;

FIG. 9 is a block diagram showing a configuration of a filter circuitaccording to Embodiment 3 of the present invention;

FIG. 10 is a block diagram showing a configuration of a filter circuitaccording to Embodiment 4 of the present invention;

FIG. 11 is a block diagram showing a configuration of a radio apparatusaccording to Embodiment 5 of the present invention;

FIG. 12 is a block diagram showing a configuration of a radio apparatusaccording to Embodiment 6 of the present invention;

FIG. 13 is a block diagram showing a configuration of a radio apparatusaccording to Embodiment 7 of the present invention; and

FIG. 14 is a block diagram showing a configuration of a radio apparatusaccording to Embodiment 8 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference now to the attached drawings, embodiments of the presentinvention will be explained below.

Embodiment 1

FIG. 1 is a block diagram showing a configuration of a filter circuit108 according to Embodiment 1 of the present invention.

A signal input terminal 101 in FIG. 1 receives an input signal and sendsit to a distributor 103. Signal output terminals 102 output outputsignals from a differential amplifier 107. The distributor 103distributes a signal in an arbitrary frequency band input from thesignal input terminal 101 into two lines of distributed signals of theequal amplitude and equal phase and outputs the respective distributedsignals to a buffer 104 and a buffer 105. The buffer 104 suppressesinterference between the two lines of distributed signals input from thedistributor 103 and outputs the signal to the differential amplifier107. The buffer 105 suppresses interference between the two lines ofdistributed signals input from the distributor 103 and outputs thesignal to the filter 106. The filter 106 applies a frequency selectionto the distributed signal input from the buffer 105 and output thefrequency-selected signal to the differential amplifier 107. Thedifferential amplifier 107 outputs a difference in amplitude componentsbetween the distributed signal input from the buffer 104 to anon-inverted input terminal and the distributed signal input from thefilter 106 to an inverted input terminal to the signal output terminals102.

The operation of the filter circuit 108 in the above describedconfiguration will be explained using FIG. 2 to FIG. 6.

First, a case where the attenuation characteristic of the filter 106 isof a low pass type, high pass type or band pass type will be explained.

First, a signal made up of frequency components f1, f2, f3 of power P1is input from the signal input terminal 101 as shown in FIG. 2. Then,the signal is distributed by the distributor 103 into portions with thesame amplitude, with power P2 (P1>P2) and amplitude reduced to half asshown in FIG. 3 and one of those signal outputs is subjected to afrequency selection at the filter 106. As a result, when the filter 106is of a low pass type, an output signal with power descending in orderof f1, f2, f3 as shown in FIG. 4A is obtained. On the other hand, whenthe filter 106 is of a high pass type, an output signal with powerdescending in order of f3, f2, f1 as shown in FIG. 4B is obtained.Furthermore, when the filter 106 is of a band pass type, an outputsignal with f2 having the highest power and the power of f1, f3 smallerthan the power of f2 is obtained as shown in FIG. 4C. On the other hand,the other of the signal outputs of the distributor 103 is not subjectedto any frequency selection, and therefore the signal in FIG. 3 is inputto the non-inverted input terminal of the differential amplifier 107 andthe signal in FIG. 4 is input to the inverted input terminal with thesame phase. Because of an in-phase signal elimination action of thedifferential amplifier 107, an output signal representing a differencein amplitude components between the distributed signals distributed bythe distributor 103 and input to the differential amplifier 107 as shownin FIG. 5 appears at the signal output terminals 102. That is, when alow pass type filter is used for the filter 106, an output signal with alow band amplitude component eliminated and having power descending inorder of f3, f2, f1 is obtained as shown in FIG. 5A. On the other hand,when a high pass type filter is used for the filter 106, an outputsignal with the high band amplitude component eliminated and amplitudecomponents having power descending in order of f1, f2, f3 is obtained asshown in FIG. 5B. Furthermore, when a band pass type filter is used forthe filter 106, an output signal with f2 having the smallest power andthe power of f1, f3 being greater than the power of f2 is obtained asshown in FIG. 5C. The largest output signal of each frequency of thedifferential amplifier 107 shown in FIG. 5 becomes P3 (P2>P3 if the gainis less than 0 [dB] and P2<P3 if the gain is equal to or greater than 0[dB]).

Then, the case where the attenuation characteristic of the filter 106 isof a band-reject type will be explained.

As shown in FIG. 2, a signal made up of frequency components f1, f2, f3of power P1 is input from the signal input terminal 101. Then, thesignal is distributed by the distributor 103 into portions with the sameamplitude, with power P2 (P1>P2) and amplitude reduced to half as shownin FIG. 3 and one of these signal outputs is subjected to a frequencyselection at the filter 106. As a result, an output signal with f2having the lowest power and the power of f1, f3 being greater than thepower of f2 as shown in FIG. 6 is obtained. On the other hand, the otherof the signal outputs of the distributor 103 is not subjected to anyfrequency selection, and therefore the signal in FIG. 3 is input to thenon-inverted input terminal of the differential amplifier 107 and thesignal in FIG. 6 is input to the inverted input terminal with the samephase. Because of an in-phase signal elimination action of thedifferential amplifier 107, an output signal representing the differencein amplitude components between the distributed signals distributed bythe distributor 103 and input to the differential amplifier 107 as shownin FIG. 7 appears at the signal output terminals 102. That is, an outputsignal with f2 having the largest power and the power of f1, f3 beingsmaller than the power of f2 as shown in FIG. 7 is obtained. Here, thelargest output signal of the differential amplifier 107 shown in FIG. 7becomes P3 (P2>P3 if the gain is less than 0 [dB] and P2<P3 if the gainis equal to or greater than 0 [dB]).

Therefore, when the rejection band of the filter 106 is set to a desiredfrequency band and the filter circuit 108 is applied to the receptionsection, the pass band of the filter 106 is set to a disturbing wavefrequency band and when the filter circuit 108 is applied to atransmission section and local oscillation section, the pass band of thefilter 106 is set to an unnecessary frequency component band, and it isthereby possible to extract only a desired frequency component.

The output signal from the signal output terminals 102 exists in therejection band of the filter 106 and even if some loss is produced whenthe received signal of the rejection band is rejected, this loss has aminor influence on the received signal in the pass band.

Thus, according to this Embodiment 1, an input signal is distributedinto two types, a difference in amplitude components between onedistributed signal which is frequency-selected by a filter and the otherdistributed signal which is not frequency-selected is output from thedifferential amplifier, and it is therefore possible to maintain theattenuation characteristic or passage characteristic in a high frequencyband. Furthermore, according to this Embodiment 1, when a band-rejecttype filter, which rejects a predetermined band, is used for the filter106, it is possible to realize integration of a band pass typeconfiguration having a steep attenuation characteristic in a highfrequency band. Furthermore, this Embodiment 1 provides the buffers 104,105 between the two output terminals of the distributor 103 and thefollowing circuit and suppresses interference between two output signalsfrom the distributor 103, and can thereby improve the stability of theattenuation characteristic.

According to Embodiment 1, a distributed signal which has not passedthrough the filter is input to the inverted input terminal of thedifferential amplifier 107 and a distributed signal which has passedthrough the filter is input to the non-inverted input terminal, but thepresent invention is not limited to this and can also be adapted so thatthe polarities of the input terminals of the differential amplifier 107are inverted and the distributed signal which has not passed through thefilter is input to the non-inverted input terminal and the distributedsignal which has passed through the filter is input to the invertedinput terminal.

Embodiment 2

FIG. 8 is a block diagram showing a configuration of a filter circuit800 according to Embodiment 2 of the present invention.

The filter circuit 800 in this Embodiment 2 corresponds to the filtercircuit 108 in Embodiment 1 shown in FIG. 1 with an adjustment circuit801, a comparator 802, a reference signal source 803 and a switch 804shown in FIG. 8 added. In FIG. 8, the same components as those in FIG. 1are assigned the same reference numerals and explanations thereof willbe omitted.

In FIG. 8, when the adjustment circuit 801 operates as a calibrationcircuit, it gives a distributed signal input from the buffer 104 a phaserotation or amplitude attenuation equivalent to an amount of phaserotation or amount of amplitude attenuation which the filter 106 givesto a signal in the pass band and outputs the resultant signal to thecomparator 802 and differential amplifier 107. Furthermore, when theadjustment circuit 801 does not operate as a calibration circuit, itgives a predetermined amount of phase rotation to the distributed signalinput from the buffer 104, and can thereby make the band of the outputsignal output from the differential amplifier 107 variable. That is, theadjustment circuit 801 sets the amount of phase rotation to be given tothe distributed signal input from the buffer 104 to a predeterminedvalue, and can thereby make the band of the output signal output fromthe differential amplifier 107 variable.

When the adjustment circuit 801 operates as the calibration circuit, thecomparator 802 detects a phase difference or amplitude differencebetween in-phase input signals to the differential amplifier 107 at thetime of calibration, outputs a first control signal for indicating anamount of calibration of the adjustment circuit 801 to the adjustmentcircuit 801 and thereby adjusts the amount of calibration of theadjustment circuit 801 and stores calibration data after the calibrationis completed. On the other hand, when the adjustment circuit 801 doesnot operate as the calibration circuit, the comparator 802 detects aphase difference between in-phase input signals to the differentialamplifier 107 at the time of adjustment of the amount of phase rotationand outputs a first control signal for setting an amount of phaserotation so that the band of the output signal output from thedifferential amplifier 107 becomes a predetermined band to theadjustment circuit 801, and thereby adjusts the amount of phase rotationset by the adjustment circuit 801 and stores the amount of phaserotation set after the adjustment of the amount of phase rotation iscompleted.

More specifically, in both cases where the adjustment circuit 801operates as the calibration circuit and where the adjustment circuit 801does not operate as the calibration circuit, the comparator 802 comparesthe voltage (first voltage) at the midpoint (connection midpoint) of thetransmission path connecting the adjustment circuit 801 and thenon-inverted input terminal of the differential amplifier 107 with thevoltage (second voltage) at the midpoint (connection midpoint) of thetransmission path connecting the filter 106 and the inverted inputterminal of the differential amplifier 107 and adjusts the amount ofcalibration or the amount of phase rotation of the adjustment circuit801 so that the voltage difference falls below a predetermined value.Furthermore, the comparator 802 controls switching of the switch 804 inboth cases where the adjustment circuit 801 operates as the calibrationcircuit and where the adjustment circuit 801 does not operate as thecalibration circuit. When the frequency of the input signal to thedistributor 103 is high and exceeds an allowable comparison operationrange of the comparator 802, a frequency division circuit is providedinside the comparator 802 to reduce the comparison frequency.

Here, the “time of calibration” means an adjustment stage for setting aphase rotation or amplitude attenuation equivalent to the amount ofphase rotation or amount of amplitude attenuation that the filter 106gives to a signal in the pass band, the “time of completion ofcalibration” means a stage at which the setting of the phase rotation oramplitude attenuation is completed, the “time of adjustment of theamount of phase rotation” means an adjustment stage for setting theamount of phase rotation so as to obtain an output signal of a desiredband from the differential amplifier 107 and the “time at whichadjustment of the amount of phase rotation is completed” means a stageat which the setting of the amount of phase rotation is completed. Whenthe adjustment circuit 801 operates as the calibration circuit, at thetime of calibration, the comparator 802 repeatedly detects the phasedifference or amplitude difference every time a distributed signal isinput from the adjustment circuit 801 and filter 106 and adjusts theamount of calibration so that the detected phase difference or amplitudedifference is reduced to a predetermined value.

On the other hand, when the adjustment circuit 801 does not operate asthe calibration circuit, at the time of adjustment of the amount ofphase rotation, the comparator 802 repeatedly detects the phasedifference every time a distributed signal is input from the adjustmentcircuit 801 and filter 106 and adjusts the amount of phase rotationuntil the phase difference becomes a predetermined value so that anoutput signal in a predetermined band is obtained. At the time ofcalibration of the adjustment circuit 801 and at the time of adjustmentof the amount of phase rotation, the reference signal source 803 sweepsthe frequency at a certain output level and outputs the frequency to thedistributor 103. Based on the control by the comparator 802, the switch804 short-circuits between the signal input terminal 101 and distributor103 when the calibration is completed and when the adjustment of theamount of phase rotation is completed and short-circuits between thereference signal source 803 and distributor 103 at the time ofcalibration and at the time of adjustment of the amount of phaserotation.

Note that the amplitude component output from the differential amplifier107 is discarded at the time of calibration and at the time ofadjustment of the amount of phase rotation and the comparator 802 isplaced in a nonoperating state and no signal is input to the comparatorwhen the calibration is completed and when the adjustment of the amountof phase rotation is completed.

Based on the considerations in the process led to the present invention,when the phase error or amplitude error of an in-phase input signal tothe two input terminals of the differential amplifier 107 is equal to orhigher than a predetermined value, the in-phase signal eliminationeffect is known to reduce and it can be said that it is important toreduce the phase error and amplitude error in realizing a steepattenuation characteristic of the filter circuit 800.

Thus, a method and procedure for calibrating the phase error andamplitude error at a high degree of accuracy will be explained.

In an ideal environment and when there is no variation in the componentsof the filter circuit 800, the adjustment circuit 801 is set so as togive the amount of phase rotation and amount of amplitude attenuationequivalent to those produced at the filter 106.

When the filter circuit 800 is made up of discrete components, theadjustment circuit 801 can be adjusted for every component which isapplied to the filter 106, whereas when the components of the filtercircuit 800 are formed on the same semiconductor substrate, individualadjustments cannot be made according to variations in thecharacteristics of the components, and therefore this Embodiment 2initially changes the input path of the signal to the distributor 103 tothe reference signal source 803 side using the switch 804 when power tothe filter circuit 800 is turned on, monitors a phase error andamplitude error between two signals input to the differential amplifier107 using the comparator 802 and controls the adjustment circuit 801 insuch a way that the phase error and amplitude error converge within apredetermined error range. Note that when power is turned on from thenext time onward, it is possible to use either a method of repeating acalibration operation again or a method of storing a first controlsignal output to the calibration circuit at the time of calibration atthe comparator 802 and fixing the state of the adjustment circuit 801and the adjustment circuit 801 is configured so as to be able to handlethe above described two types of situations.

Then, in the case of an environment variation such as a temperaturevariation, the amount of phase rotation and the amount of amplitudeattenuation produced at the filter 106 may be changed from the valuewhen power is turned on, and therefore it is possible to improve thecalibration accuracy by carrying out calibration similar to that whenpower is turned on at a timing at which the filter circuit 800 is set toa nonoperating state. The rest of the operation is the same as that inEmbodiment 1, and therefore explanations thereof will be omitted.Moreover, the respective output signals are the same as those in FIG. 2to FIG. 7, and therefore explanations thereof will be omitted.

Then, a case where the adjustment circuit 801 gives an amount ofamplitude attenuation equivalent to that in the pass band of the filter106 and adjusts the amount of phase rotation will be explained.

When the amount of phase rotation given by the adjustment circuit 801 isequivalent to that in the pass band of the filter 106, the frequencycharacteristics of the filter 106 and filter circuit 800 are inverted asalready explained. Furthermore, the cutoff frequencies (3 dB attenuationpoint) of the filter 106 and the filter circuit 800 are the same.

When the amount of phase rotation given by the adjustment circuit 801 isdifferent from that with the same frequency in the pass band of thefilter 106, the in-phase elimination effect of the differentialamplifier 107 cannot be obtained and even the area within the pass bandof the filter 106 is no longer the rejection band of the filter circuit800. Therefore, the cutoff frequency of the filter circuit 800 can bemade variable.

In addition to the effects of aforementioned Embodiment 1, thisEmbodiment 2 suppresses the phase error or amplitude error of anin-phase input signal to the two input terminals of the differentialamplifier 107 to a value less than a predetermined value using thereference signal source 803 for calibration, adjustment circuit 801 andcomparator 802, and can thereby realize an extremely steep attenuationcharacteristic. Furthermore, this Embodiment 2 short-circuits betweenthe reference signal source and the distributor at the time ofcalibration by the adjustment circuit 801 and sweeps the outputfrequency of the reference signal source, and can thereby realizecalibration at a higher degree of accuracy.

When the adjustment circuit 801 does not operate as the calibrationcircuit, this Embodiment 2 only adjusts the amount of phase rotation,but the present invention is not limited to this and can also be adaptedso that the adjustment circuit 801 adjusts both the amount of amplitudeattenuation and amount of phase rotation, gives the phase rotation andamplitude attenuation equivalent to the amount of phase rotation andamount of amplitude attenuation given by the filter 106 to the signal inthe pass band to the distributed signal input from the buffer 104 andadjusts the band of the output signal output from the differentialamplifier 107 simultaneously. In this case, the adjustment circuit foradjusting the band of the output signal may be different from thecalibration circuit for giving the amount of phase rotation and amountof amplitude attenuation or one circuit may be shared in place of thetwo circuits.

Embodiment 3

FIG. 9 is a block diagram showing a configuration of a filter circuit900 according to Embodiment 3 of the present invention.

The filter circuit 900 according to this Embodiment 3 corresponds to thefilter circuit 800 according to Embodiment 2 shown in FIG. 8 with aselector 903 added, the filter 106 replaced by a filter 901 and theadjustment circuit 801 replaced by an adjustment circuit 902 as shown inFIG. 9. In FIG. 9, the same components as those in FIG. 1 and FIG. 8 areassigned the same reference numerals and explanations thereof will beomitted.

In FIG. 9, the filter 901 has a band-reject type configuration made upof an inductor and a capacitance and makes the rejection band variableby making the inductor value or capacitance value variable. That is, thefilter 901 rejects passage of some frequency components in apredetermined communication band and allows only predetermined frequencycomponents to pass. Then, the filter 901 outputs the frequencycomponents in the pass band other than the rejection band within thepass band of the distributed signal input from the buffer 105 to thedifferential amplifier 107. When the adjustment circuit 902 operates asthe calibration circuit, the adjustment circuit 902 gives a phaserotation, amplitude attenuation equivalent to the amount of phaserotation, amount of amplitude attenuation given by the filter 901 to asignal in the pass band to the output signal from the buffer 104 andoutputs the output signal to the differential amplifier 107 andcomparator 802. On the other hand, when the adjustment circuit 902 doesnot operate as the calibration circuit, the adjustment circuit 902 givesa predetermined amount of phase rotation to the distributed signal inputfrom the buffer 104, and can thereby make the band of the output signaloutput from the differential amplifier 107 variable. That is, theadjustment circuit 902 sets the amount of phase rotation to be given tothe distributed signal input from the buffer 104 to a predeterminedamount of phase rotation, and can thereby make the band of the outputsignal output from the differential amplifier 107 variable as in thecase of making the band of the distributed signal passing through thefilter 106 variable. The selector 903 outputs to the adjustment circuit902 a second control signal for controlling the rejection band of thefilter 901 based on a band selection signal for selecting the band ofthe filter 901 input from a detection circuit (not shown) and at thesame time controlling the amount of phase rotation and amount ofamplitude attenuation of the adjustment circuit 902 according to avariation in the amount of phase rotation and amount of amplitudeattenuation caused by the change of the rejection band of the filter901. Note that the amplitude component output from the differentialamplifier 107 is discarded at the time of calibration and the comparator802 is set to a nonoperating state and no signal is input to thecomparator 802 when the calibration is completed.

The operation of the filter circuit 900 configured as described abovewill be explained below.

Through the user's manual selection of a radio set to which the filtercircuit 900 is applied or automatic selection based on the receptionfield intensity detected by the radio set and base station controlchannel information, a band selection signal is input to the selector903. A band variable signal is output from the selector 903 whichreceived the band selection signal to the filter 901 and the rejectionband of the filter 901 is fixed.

When a band selection signal, which selects a band different from theprevious band, is received, the amount of phase rotation and amount ofamplitude attenuation in the pass band are also changed at the filter901 caused by the change of the rejection band. When the adjustmentcircuit 902 gives a fixed amount of phase rotation and amount ofamplitude attenuation to the input signal, the filter circuit 900 cannotmaintain the attenuation characteristic, and therefore the adjustmentcircuit 902 stores input signal calibration data corresponding to theamount of phase rotation and amount of amplitude attenuation in the passband of the filter 901 which vary according to the band variable signalor provides an individual circuit corresponding to each amount of phaserotation and amount of amplitude attenuation, and the selector 903changes the state of the adjustment circuit 902 according to the secondcontrol signal according to the band variable signal. After the abovedescribed change is completed, the phase error or amplitude error of thein-phase input signal to the two input terminals of the differentialamplifier 107 is suppressed to less than a predetermined value throughthe same process as that in Embodiment 2. Furthermore, when theadjustment circuit 902 does not operate as the calibration circuit, theadjustment circuit 902 gives a predetermined amount of phase rotation tothe distributed signal input from the buffer 104, and can thereby changethe band of the output signal output from the differential amplifier 107in the same way that the filter 901 changes the rejection band. The restof the operation is the same as that in Embodiment 1 and Embodiment 2and therefore explanations thereof will be omitted. Furthermore, therespective output signals are the same as those in FIG. 2, FIG. 3, FIG.6 and FIG. 7 and therefore explanations thereof will be omitted.

As shown above, in addition to the effects of Embodiment 1 andEmbodiment 2, this Embodiment 3 adopts a band-reject type configurationfor the filter 901, and can thereby integrate the band pass type filter,which can make the pass band variable. Furthermore, this Embodiment 3controls the rejection band of the filter 901 and at the same timecontrols the amount of phase rotation and amount of amplitudeattenuation of the adjustment circuit 902 according to the rejectionband of the filter 901, and can thereby flexibly handle changes in thedesired band and realize a steeper attenuation characteristic.Furthermore, this Embodiment 3 can change the band of the output signaloutput from the differential amplifier 107 using both the filter 901 andadjustment circuit 902, and can thereby obtain the output signal in adesired band at a high speed.

According to this Embodiment 3, the band selection signal is input fromthe outside of the filter circuit 900, but the present invention is notlimited to this and can also be adapted so that a function of generatinga band selection signal is incorporated as the function of the filtercircuit 900, a band selection signal generation section is provided andthe band selection signal is input to the selector 903 from the bandselection signal generation section inside the filter circuit 900.

Embodiment 4

FIG. 10 is a block diagram showing a configuration of a filter circuitaccording to Embodiment 4 of the present invention.

The filter circuit 1000 according to this Embodiment 4 corresponds tothe filter circuit 108 according to Embodiment 1 shown in FIG. 1 with adetection circuit 1001, a switch 1002, a switch 1005 and a switch 1006added as shown in FIG. 10 and the differential amplifier 107 replaced bya differential amplifier 1007. Note that in FIG. 10, the same componentsas those in FIG. 1 are assigned the same reference numerals andexplanations thereof will be omitted.

In FIG. 10, the detection circuit 1001 detects disturbing waves orunnecessary frequency components other than a desired frequency band,and when any disturbing wave or unnecessary frequency component isdetected, the detection circuit 1001 changes the switch 1002 so that thesignal input terminal 101 and the distributor 103 are connected and whenno disturbing wave or unnecessary frequency component is detected, thedetection circuit 1001 changes the switch 1002 so that the signal inputterminal 101 and the non-inverted input terminal of the differentialamplifier 1007 are connected. The switch 1002 has a switch terminal 1003and a switch terminal 1004, short-circuits between the signal inputterminal 101 and the switch terminal 1003 or the switch terminal 1004based on the detection result of the detection circuit 1001 and therebychanges the path when the signal input terminal 101 is connected to thenon-inverted input terminal of the differential amplifier 1007 and whenthe signal input terminal 101 is connected to the distributor 103. Theswitch 1005 changes the path when the buffer 104 is connected to thenon-inverted input terminal of the differential amplifier 1007 and whenthe buffer 104 is connected to no circuit based on the detection resultof the detection circuit 1001. The switch 1006 changes the path when theinverted input terminal of the differential amplifier 1007 is groundedand when the inverted input terminal is connected to the filter 106based on the detection result of the detection circuit 1001. Thedifferential amplifier 1007 outputs a difference in amplitude componentsbetween the distributed signal input from the buffer 104 to thenon-inverted input terminal and the distributed signal input from thefilter 106 to the inverted input terminal or a difference in amplitudecomponents between the input signal input from the signal input terminal101 to the non-inverted input terminal and the grounded inverted inputterminal to the signal output terminals 102. Furthermore, thedifferential amplifier 1007 changes the gain based on the detectionresult at the detection circuit 1001. That is, the differentialamplifier 1007 gives the gain for complementing the attenuation in theinput signal produced when the input signal is distributed by thedistributor 103 to the input distributed signal.

The operation of the filter circuit configured as shown above will beexplained below.

When the detection circuit 1001 detects no disturbing wave orunnecessary frequency component, the switch 1002 short-circuits betweenthe switch terminal 1003 and the signal input terminal 101, the switch1005 disconnects the input terminal from the output terminal and theswitch 1006 short-circuits between the input terminal and the outputterminal and grounds the inverted input terminal of the differentialamplifier 1007 and at the same time sets the gain of the differentialamplifier 1007 to G1[dB]. On the other hand, when the detection circuit1001 detects a disturbing wave or unnecessary frequency component, theswitch 1002 short-circuits between the switch terminal 1004 and thesignal input terminal 101, the switch 1005 short-circuits between theinput and output terminals, and the switch 1006 disconnects the inputterminal from the output terminal and at the same time sets the gain ofthe differential amplifier 1007 to G2 (G2=G1+3)[dB] to complement theattenuation of the filter 106 to thereby suppress a level variation atthe signal output terminal 102 at the time of switching of the circuit.The rest of the operation is the same as that in Embodiment 1 andexplanations thereof will be omitted. Furthermore, the respective outputsignals are the same as those in FIG. 2 to FIG. 7 and thereforeexplanations thereof will be omitted.

Thus, in addition to the effects in Embodiment 1, in an environment inwhich no disturbing wave or unnecessary frequency component exists, thisEmbodiment 4 changes the switch 1002 so that the signal input terminal101 is directly connected to the non-inverted input terminal of thedifferential amplifier 1007 to switch between theoperation/non-operation of the function of the filter 106, preventingthe received signal at a desired frequency from attenuating, and canthereby suppress the gain of the differential amplifier 1007 and reducethe current consumption. Furthermore, when the detection circuit 1001detects a frequency component other than the desired frequency, thisEmbodiment 4 changes the setting of the differential amplifier 1007 tothe gain for complementing the attenuation at the time of signaldistribution, and can thereby keep the output level constant.

In this Embodiment 4, the distributed signal output from the buffer 104is directly input to the non-inverted input terminal of the differentialamplifier 1007, but the present invention is not limited to this, and asin the case of Embodiment 2, it is also possible to add a calibrationcircuit for suppressing a phase error or amplitude error of an in-phaseinput signal to the two input terminals of the differential amplifier1007 to less than a predetermined value between the buffer 104 thenon-inverted input terminal of the differential amplifier 1007 or adopta band-reject type filter whose rejection band is variable for thefilter 106 and at the same time add a calibration circuit forsuppressing the phase error or amplitude error of the in-phase inputsignal to the two input terminals of the differential amplifier 1007 toless than a predetermined value or a selector for changing the band ofthe filter 106 in a variable manner. This allows a steeper attenuationcharacteristic to be realized for the filter circuit. Furthermore, inthis Embodiment 4, the band of the distributed signal that passesthrough the filter 106 is fixed, but the present invention is notlimited to this and can be adapted so that a band rejection typeconfiguration is used for the filter 106 and a selector is provided asin the case of Embodiment 3 and the band for rejecting the passage ofthe filter 106 is made variable. Furthermore, when the detection circuit1001 detects a disturbing wave or unnecessary frequency component, thisEmbodiment 4 increases the gain of the differential amplifier 1007 by 3[dB], but the present invention is not limited to this and the gain canbe set to an arbitrary value.

Embodiment 5

FIG. 11 is a block diagram showing a configuration of a radio apparatus1100 according to Embodiment 5 of the present invention. A low-noiseamplifier 1101, a filter circuit 1102 and a mixer 1103 constitute ahigh-frequency section 1104.

In FIG. 11, the low-noise amplifier 1101 amplifies a received signalreceived by the antenna 1105 and outputs the amplified signal to thefilter circuit 1102. The filter circuit 1102 allows a desired frequencycomponent of the received signal input from the low-noise amplifier 1101to be passed and output to the mixer 1103. Here, for the filter circuit1102, any one of the filter circuit 108 in FIG. 1, filter circuit 800 inFIG. 8, filter circuit 900 in FIG. 9 and filter circuit 1000 in FIG. 10can be applied as appropriate. The mixer 1103 converts the frequency ofthe received signal input from the filter circuit 1102 and outputs thereceived signal to the demodulator 1106. The demodulator 1106demodulates the received signal input from the mixer 1103 and obtainsreception data.

The operation of the radio apparatus 1100 configured as shown above willbe explained below.

A disturbing wave or unnecessary frequency component other than adesired frequency band existing at the output of the low-noise amplifier1101 is attenuated by the filter circuit 1102 to prevent saturation ofthe mixer 1103. The operation of the filter circuit 1102 is the same asthat described in any one of Embodiment 1 to Embodiment 4 andexplanations thereof will be omitted.

In addition to the effects of Embodiment 1 to Embodiment 4, thisEmbodiment 5 attenuates a disturbing wave or unnecessary frequencycomponent other than a desired frequency band existing at the output ofthe low-noise amplifier 1101 using the filter circuit 1102 and preventssaturation of the mixer 1103, and can thereby suppress deterioration ofthe sensitivity of the demodulator 1106. Furthermore, by adopting thesame configuration as that of any one of the filter circuits 108, 800,900, 1000 for the filter circuit 1102, this Embodiment 5 facilitates theformation of the high-frequency section 1104 on the same semiconductorsubstrate because all these filter circuits have a high degree ofintegration and affinity, and can thereby realize miniaturization of theradio apparatus 1100 and reduce the number of parts.

In this Embodiment 5, the filter circuit 1102 is interposed between thelow-noise amplifier 1101 and mixer 1103, but the present invention isnot limited to this and can also be adapted so that the filter circuit1102 is disposed at a position other than between the low-noiseamplifier 1101 and mixer 1103 in the high-frequency section 1104 toenhance integration of the filter circuit in a high-frequency band andobtain a steep attenuation characteristic. Furthermore, in thisEmbodiment 5, the filter circuit 1102 is used at the high-frequencysection 1104, but the present invention is not limited to this and thefilter circuit 1102 can be used in a low frequency section, etc., otherthan the high-frequency section 1104.

Embodiment 6

FIG. 12 is a block diagram showing a configuration of a radio apparatus1200 according to Embodiment 6 of the present invention. A modulator1201, a filter circuit 1202 and a power amplifier 1203 constitute ahigh-frequency section 1204.

In FIG. 12, the modulator 1201 modulates a transmission signal inputfrom the baseband section 1206 and outputs the modulated signal to thefilter circuit 1202. The filter circuit 1202 allows a desired frequencycomponent of the transmission signal input from the modulator 1201 topass and outputs the frequency component to the power amplifier 1203.Here, any one of the filter circuit 108 in FIG. 1, the filter circuit800 in FIG. 8, the filter circuit 900 in FIG. 9 and the filter circuit1000 in FIG. 10 can be used for the filter circuit 1202 as appropriate.The power amplifier 1203 amplifies transmit power in the transmissionsignal input from the filter circuit 1202 and transmits the transmissionsignal from the antenna 1205. The baseband section 1206 processestransmission data into a baseband, generates a transmission signal andoutputs the transmission signal generated to the modulator 1201.

The operation of the radio apparatus 1200 configured as shown above willbe explained below.

The unnecessary frequency component other than a desired frequency bandexisting at the output of the modulator 1201 is attenuated at the filtercircuit 1202. The operation of the filter circuit 1202 is the same asthose of Embodiment 1 to Embodiment 4 and explanations thereof will beomitted.

In addition to the effects of Embodiment 1 to Embodiment 4, thisEmbodiment 6 attenuates the unnecessary frequency component other than adesired frequency band existing at the output of the modulator 1201using the filter circuit 1202, and can thereby suppress unnecessaryradiation from the antenna 1205. Furthermore, by adopting the sameconfiguration as that of any one of the filter circuits 108, 800, 900,1000 for the filter circuit 1202, this Embodiment 6 facilitates theformation of the high-frequency section 1204 on the same semiconductorsubstrate because all these filter circuits have a high degree ofintegration and affinity, and can thereby realize miniaturization of theradio apparatus 1200 and reduce the number of parts.

In this Embodiment 6, the filter circuit 1202 is interposed between themodulator 1201 and power amplifier 1203, but the present invention isnot limited to this and can also be adapted so that the filter circuit1202 is disposed at a position other than between the modulator 1201 andpower amplifier 1203 in the high-frequency section 1204 to enhanceintegration of the filter circuit in a high-frequency band and obtain asteep attenuation characteristic. Furthermore, in this Embodiment 6, thefilter circuit 1202 is used at the high-frequency section 1204, but thepresent invention is not limited to this and the filter circuit 1202 canalso be used in a low frequency section other than the high-frequencysection 1204.

Embodiment 7

FIG. 13 is a block diagram showing a configuration of a radio apparatus1300 according to Embodiment 7 of the present invention.

The radio apparatus 1300 according to this Embodiment 7 corresponds tothe radio apparatus 1100 according to Embodiment 5 shown in FIG. 11 witha local oscillator 1301 and a filter circuit 1302 added as shown in FIG.13. In FIG. 13, the same components as those in FIG. 11 are assigned thesame reference numerals and explanations thereof will be omitted.

The low-noise amplifier 1101, filter circuit 1102, mixer 1103, localoscillator 1301 and filter circuit 1302 constitute a high-frequencysection 1303.

In FIG. 13, the local oscillator 1301 generates a signal with apredetermined frequency for frequency conversion and outputs the signalto the filter circuit 1302. The filter circuit 1302 allows a desiredfrequency component of the signal for frequency conversion input fromthe local oscillator 1301 to pass and outputs the desired frequencycomponent to the mixer 1103. Here, any one of the filter circuit 108 inFIG. 1, the filter circuit 800 in FIG. 8, the filter circuit 900 in FIG.9 and the filter circuit 1000 in FIG. 10 can be used for the filtercircuit 1302 as appropriate.

In the radio apparatus 1300 configured as shown above, the unnecessaryfrequency component other than a desired frequency band existing at theoutput of the local oscillator 1301 is attenuated at the filter circuit1302.

Thus, in addition to the effects of Embodiment 1 to Embodiment 4, thisEmbodiment 7 attenuates the unnecessary frequency component other thanthe desired frequency components existing at the output of the localoscillator 1301 using the filter circuit 1302, and can thereby suppressmutual modulation distortion produced at the mixer 1103 and suppressdeterioration of the sensitivity of the demodulator 1106. Furthermore,by adopting the same configuration as that of any one of the filtercircuits 108, 800, 900, 1000 for the filter circuit 1302, thisEmbodiment 7 facilitates the formation of the high-frequency section1303 on the same semiconductor substrate because all these filtercircuits have a high degree of integration and affinity, and can therebyrealize miniaturization of the radio apparatus 1300 and reduce thenumber of parts.

In this Embodiment 7, the filter circuit 1102 is interposed between thelow-noise amplifier 1101 and mixer 1103 in the high frequency section1303 and the filter circuit 1302 is interposed between the localoscillator 1301 and mixer 1103 in the high frequency section 1303, butthe present invention is not limited to this and can also be adapted sothat the filter circuit 1102 is disposed at a position other thanbetween the low-noise amplifier 1101 and mixer 1103 in the highfrequency section 1303 or the filter circuit 1302 is disposed at aposition other than between the local oscillator 1301 and mixer 1103 inthe high-frequency section 1303 to enhance integration of the filtercircuit in a high-frequency band and obtain a steep attenuationcharacteristic. Furthermore, in this Embodiment 7, the filter circuits1102, 1302 are used at the high-frequency section 1303, but the presentinvention is not limited to this and the filter circuits 1102, 1302 canalso be used in a low frequency section other than the high-frequencysection 1303.

Embodiment 8

FIG. 14 is a block diagram showing a configuration of a radio apparatus1400 according to Embodiment 8 of the present invention. A distributor1403, a ½ frequency divider 1404, a differential amplifier 1405, an evenharmonics mixer 1406, an even harmonics mixer 1407, a filter circuit1408 and a filter circuit 1409 constitute a frequency conversion circuit1410. Furthermore, orthogonal demodulators 1411 a, 1411 b constitute areceiver 1412. Furthermore, orthogonal demodulators 1414 a, 1414 bconstitute a receiver 1415. Here, the receiver 1412 is a directconversion receiver using a radio frequency of 2.5×f0 and the receiver1415 is a direct conversion receiver using a radio frequency of f0.

In FIG. 14, the local oscillator 1401 generates a frequency of f0 andoutputs it to the distributor 1402. The distributor 1402 distributes thesignal with the frequency f0 input from the local oscillator 1401 andoutputs the distributed signals to the distributor 1403 and phaseshifter 1413. The distributor 1403 distributes the signal input from thedistributor 1402 and outputs the distributed signal to the ½ frequencydivider 1404 and the non-inverted input terminal of the differentialamplifier 1405. The ½ frequency divider 1404 divides the frequency ofthe signal input from the distributor 1403 to ½ and outputs it to theeven harmonics mixer 1406 and even harmonics mixer 1407. The invertedinput terminal of the differential amplifier 1405 is grounded and thedifferential amplifier 1405 outputs the distributed signal input fromthe distributor 1403 to the even harmonics mixers 1406, 1407. The evenharmonics mixer 1406 mixes a frequency (2×f0) doubling the outputfrequency of the differential amplifier 1405 and the output frequency(0.5×f0) of the ½ frequency divider 1404 and outputs the mixed frequencyto the filter circuit 1408. The even harmonics mixer 1407 mixes afrequency (2×f0) doubling the output frequency of the differentialamplifier 1405 and the output frequency (0.5×f0) of the ½ frequencydivider 1404 and outputs the mixed frequency to the filter circuit 1409.The filter circuit 1408 allows a desired frequency component of thesignal input from the even harmonics mixer 1406 to pass and outputs thedesired frequency component to the orthogonal demodulator 1411 a. Here,any one of the filter circuit 108 in FIG. 1, the filter circuit 800 inFIG. 8, the filter circuit 900 in FIG. 9 and the filter circuit 1000 inFIG. 10 can be used for the filter circuit 1408 as appropriate. Thefilter circuit 1409 allows a desired frequency component of the signalinput from the even harmonics mixer 1407 to pass and outputs the desiredfrequency component to the orthogonal demodulator 1411 b. Here, any oneof the filter circuit 108 in FIG. 1, the filter circuit 800 in FIG. 8,the filter circuit 900 in FIG. 9 and the filter circuit 1000 in FIG. 10can be used for the filter circuit 1409 as appropriate. The orthogonaldemodulator 1411 a orthogonal-demodulates the signal input from thefilter circuit 1408 to obtain reception data. The orthogonal demodulator1411 b orthogonal-demodulates the signal input from the filter circuit1409 to obtain reception data. The phase shifter 1413 generates twowaves having a phase difference of 90 degrees from the distributedsignal input from the distributor 1402 and outputs the two waves to theorthogonal demodulator 1414 a and orthogonal demodulator 1414 b. Theorthogonal demodulator 1414 a orthogonal-demodulates the signal inputfrom the phase shifter 1413 to obtain reception data. The orthogonaldemodulator 1414 b orthogonal-demodulates the signal input from thephase shifter 1413 to obtain reception data.

The operation of the radio apparatus 1400 configured as shown above willbe explained below.

At the output terminals of the even harmonics mixers 1406, 1407, adesired signal component of 2.5×f0 and unnecessary frequency componentof 1.5×f0 appear with equal amplitudes. By adopting a low pass type forthe filter 106 constituting the filter circuit 108, 800 or 1000, it ispossible to extract only a desired frequency component.

The case where any frequency component other than the aforementionedfrequency component exists as the unnecessary frequency can be handledusing a filter 901, which is the configuration of the filter 106,changed from a low pass type to a band-reject type.

Here, if f0 is assumed to be, for example, a 2 GHz band, an outputsignal of a 5 GHz band can be extracted from the frequency conversioncircuit 1410, and therefore providing a local oscillator of a 2 GHz bandmakes it possible to realize a receiver applicable to a dual-band of 2GHz band/5 GHz band. The operations of the filter circuits 1408, 1409are the same as those in Embodiment 1 to Embodiment 4 and explanationsthereof will be omitted.

Thus, in addition to the effects of Embodiment 1 to Embodiment 4, thisEmbodiment 8 can provide a radio apparatus applicable to a dual-bandsystem with unnecessary frequencies due to local oscillation frequenciesreduced.

In this Embodiment 8, the inverted input terminal of the differentialamplifier 1405 is grounded and the distributed signal is input to thenon-inverted input terminal, but the present invention is not limited tothis and can also be adapted so that the polarities of the inputterminals of the differential amplifier 1405 are inverted, thenon-inverted input terminal is grounded and the distributed signal isinput to the inverted input terminal. Furthermore, this Embodiment 8uses the differential amplifier 1405, but the present invention is notlimited to this and using a distributor instead of the differentialamplifier 1405 can also obtain the same effect. Furthermore, thisEmbodiment 8 applies the frequency conversion circuit 1410 to the directconversion receiver, but the present invention is not limited to thisand the same effect can also be obtained by applying the frequencyconversion circuit 1410 to a direct conversion transmitter.

As described above, the present invention can maintain the attenuationcharacteristic or passage characteristic in a high-frequency band usingan in-phase elimination effect of a differential amplifier.

This application is based on the Japanese Patent Application No.2002-319745 filed on Nov. 1, 2002 and the Japanese Patent ApplicationNo. 2003-152532 filed on May 29, 2003, entire content of which isexpressly incorporated by reference herein.

INDUSTRIAL APPLICABILITY

The present invention is preferably applicable to a high-frequency bandfilter circuit and radio apparatus used for a digital radiocommunication system, etc.

[FIG. 1]

-   104 BUFFER-   103 DISTRIBUTOR-   105 BUFFER-   106 FILTER    [FIG. 2]-   POWER-   FREQUENCY    [FIG. 3]-   POWER-   FREQUENCY    [FIG. 4A]-   POWER-   FREQUENCY    [FIG. 4B]-   POWER-   FREQUENCY    [FIG. 4C]-   POWER-   FREQUENCY    [FIG. 5A]-   POWER-   FREQUENCY    [FIG. 5B]-   POWER-   FREQUENCY    [FIG. 5C]-   POWER-   FREQUENCY    [FIG. 6]-   POWER-   FREQUENCY    [FIG. 7]-   POWER-   FREQUENCY    [FIG. 8]-   104 BUFFER-   801 ADJUSTMENT CIRCUIT-   103 DISTRIBUTOR-   802 COMPARATOR-   105 BUFFER-   106 FILTER    [FIG. 9]-   104 BUFFER-   902 ADJUSTMENT CIRCUIT-   103 DISTRIBUTOR-   802 COMPARATOR-   105 BUFFER-   903 SELECTOR-   BAND SELECTION SIGNAL    [FIG. 10]-   1001 DETECTION CIRCUIT-   104 BUFFER-   103 DISTRIBUTOR-   105 BUFFER-   106 FILTER    [FIG. 11]-   1102 FILTER CIRCUIT-   1106 DEMODULATOR-   RECEIVED DATA    [FIG. 12]-   1202 FILTER CIRCUIT-   1201 MODULATOR-   IQ SIGNAL-   1206 BASEBAND SECTION-   TRANSMISSION DATA    [FIG. 13]-   1102 FILTER CIRCUIT-   1106 DEMODULATOR-   RECETION DATA-   1302 FILTER CIRCUIT    [FIG. 14]-   1403 DISTRIBUTOR-   1408 FILTER CIRCUIT-   1404 ½ FREQUENCY DIVIDER-   1409 FILTER CIRCUIT-   1402 DISTRIBUTOR-   1413 PHASE SHIFTER

1. A filter circuit comprising: a distributor that distributes an inputsignal into two lines of distributed signals; a filter that applies afrequency selection to one of said distributed signals distributed bysaid distributor so as to pass in a predetermined band; and adifferential amplifier that outputs a difference in amplitude componentsbetween said other distributed signal distributed by said distributorand said one distributed signal frequency-selected by said filter. 2.The filter circuit according to claim 1, further comprising a bufferthat suppresses interference between said two lines of distributedsignals distributed by said distributor, wherein said filter applies afrequency selection to said one distributed signal whose interference issuppressed by said buffer, and said differential amplifier outputs adifference in amplitude components between said other distributed signalwhose interference is suppressed by said buffer and said one distributedsignal frequency-selected by said filter.
 3. The filter circuitaccording to claim 1, further comprising a calibration circuit thatgives an amount of phase rotation or amount of amplitude attenuationequivalent to the phase rotation or amplitude attenuation in a band inwhich said one distributed signal passes through said filter to saidother distributed signal, wherein said differential amplifier outputs adifference in amplitude components between said one distributed signalfrequency-selected by said filter and said other distributed signalgiven said amount of phase rotation or said amount of amplitudeattenuation by said calibration circuit.
 4. The filter circuit accordingto claim 3, further comprising: a reference signal source that generatesa reference signal for adjusting said amount of phase rotation or saidamount of amplitude attenuation given by said calibration circuit tosaid other distributed signal; and a switch that performs switching soas to input said reference signal to said distributor when saidcalibration circuit adjusts said amount of phase rotation or said amountof amplitude attenuation to be given to said other distributed signaland performs switching so as to input an input signal to saiddistributor when said calibration circuit gives said amount of phaserotation or said amount of amplitude attenuation to said otherdistributed signal, wherein said distributor distributes said referencesignal into two lines of signals when said calibration circuit adjustssaid amount of phase rotation or said amount of amplitude attenuation tobe given to said other distributed signal, or distributes said inputsignal into two lines of signals when said calibration circuit givessaid amount of phase rotation or said amount of amplitude attenuation tosaid other distributed signal.
 5. The filter circuit according to claim3, further comprising a comparator that compares a first voltage whichis the voltage at a midpoint of a transmission path that connects saidcalibration circuit and said differential amplifier with a secondvoltage which is the voltage at a midpoint of a transmission path thatconnects said filter and said differential amplifier, wherein saidcalibration circuit gives said amount of phase rotation or said amountof amplitude attenuation to said other distributed signal until thecomparison result of said comparator that the difference between saidfirst voltage and said second voltage falls short of a predeterminedvalue is obtained.
 6. The filter circuit according to claim 1, whereinsaid filter rejects passage of said one distributed signal in arejection band which is a band for rejecting passage and thereby appliesa frequency selection to said one distributed signal, and saiddifferential amplifier outputs a difference in amplitude componentsbetween said one distributed signal which has passed through said filterand said other distributed signal.
 7. The filter circuit according toclaim 3, wherein said filter rejects passage of said one distributedsignal in a rejection band which is a band for rejecting passage,thereby applies a frequency selection to said one distributed signal andmakes said rejection band variable, and every time said rejection bandis changed, said calibration circuit gives an amount of phase rotationand amount of amplitude attenuation equivalent to the phase rotation andamplitude attenuation in the band in which said one distributed signalpasses through said filter whose said rejection band has been changed tosaid other distributed signal.
 8. The filter circuit according to claim1, further comprising an adjustment circuit that gives a predeterminedamount of phase rotation to said other distributed signal distributed bysaid distributor and makes said amount of phase rotation to be given tosaid other distributed signal variable, wherein said differentialamplifier outputs a difference in amplitude components between saidother distributed signal given said amount of phase rotation by saidadjustment circuit and said one distributed signal whose frequency hasbeen selected by said filter.
 9. The filter circuit according to claim1, further comprising: a detection circuit that detects a frequencycomponent other than a desired frequency component; and a switch thatperforms switching so as to select a transmission path in which an inputsignal is input to said differential amplifier through said distributorwhen said detection circuit detects a frequency component other than adesired frequency component, and select a transmission path in which theinput signal is input to said differential amplifier without passingthrough said distributor when said detection circuit detects only adesired frequency component, wherein said differential amplifier outputssaid input signal as is when said detection circuit detects only thedesired frequency component, or outputs a difference in amplitudecomponents between said other distributed signal and said onedistributed signal frequency-selected by said filter when said detectioncircuit detects any frequency component other than the desired frequencycomponent.
 10. The filter circuit according to claim 9, wherein saiddifferential amplifier outputs a difference in said amplitude componentsplus a gain for complementing the attenuation in the input signalgenerated by said distributor distributing the input signal when saiddetection circuit detects a frequency component other than the desiredfrequency.
 11. A radio apparatus comprising a filter circuit, saidfilter circuit comprising: a distributor that distributes an inputsignal into two lines of distributed signals; a filter that applies afrequency selection to one of said distributed signals distributed bysaid distributor so as to pass in a predetermined band; and adifferential amplifier that outputs a difference in amplitude componentsbetween said other distributed signal distributed by said distributorand said one distributed signal frequency-selected by said filter.
 12. Amethod of selecting an output signal comprising: a step of distributingan input signal into two lines of distributed signals; a step ofapplying a frequency selection to one of said distributed signals byrejecting passage of said one distributed signal distributed in arejection band which is a band for rejecting passage; and a step ofoutputting a difference in amplitude components between said otherdistributed signal and said one frequency-selected distributed signal.